The present invention relates to a magnetizing current control circuit which operates with a magnetic recording head in a magnetic data storage and retrieval system. In particular, the present invention relates to a magnetizing current control circuit having improved switching performance and circuit component voltage breakdown prevention.
In magnetic data storage and retrieval systems, a magnetic recording head records two-logic-state data to a magnetic data storage medium, such as a magnetic tape or magnetic disc. The magnetic recording head has an inductive coil with currents provided therethrough in alternate directions, representing the data, to impart a series of alternate magnetic field patterns over time to the magnetic medium moving by it. Producing alternate magnetic field patterns over time entails switching the electric current through the inductive coil between forward and reverse directions therethrough to correspond to the data. Current flowing through the inductive coil generates a magnetic field oriented in a direction corresponding to the direction of flow through the coil; thus, reversing the direction of current reverses the orientation of the magnetic field. The magnetic fields generated by the inductive coil currents intersect the magnetic medium to polarize adjacent magnetic medium regions which in effect serve as data symbol storage positions on the medium and so form magnetic patterns along a corresponding one of more or less concentric tracks in the medium from which an information signal can be retrieved.
Controlling the directions of currents through the inductive coil is the purpose of a magnetizing current control circuit. A typical magnetizing current control circuit includes a switching network and a signal coupler. The switching network is connected to the ends of the inductive coil in the magnetic recording head at first and second head nodes, and includes four switching transistors arranged as pairs with each pair member connected to a corresponding one of these head nodes. One pair is switched on directing current flow in one direction through the inductive coil with the other pair switched off and, alternatively, this latter pair is switched on to direct current flow through the inductive coil in the opposite direction with the first pair being switched off. More specifically, the switching transistors are connected to the inductive coil such that a first switching transistor is connected between a first electrical power source and the first head node, a second switching transistor is connected between the first electrical power source and the second head node, a third switching transistor is connected between the first head node and a second electrical power source, and a fourth switching transistor is connected between the second head node and the second electrical power source.
The signal coupler, which responds to input signals, provides control signals to selectively switch the four switching transistors on and off in pairs, thereby controlling the direction of current through the inductive coil. Specifically, to direct current in one direction through the inductive coil, the signal coupler switches on the first and fourth switching transistors and switches off the second and third switching transistors. Conversely, to direct current in the opposite direction through the inductive coil, the signal coupler switches off the first and fourth switching transistors and switches on the second and third switching transistors.
One principle concern in the performance of magnetizing current control circuits is the duration of time needed to complete a switching of current direction through the inductive coil which directly affects the switching rate. Switching rate, a measure of how often the magnetizing current control circuit can reverse current direction through the inductive coil per unit of time, determines the maximum linear spatial density of data along a track in the magnetic medium. Ultimately, a higher switching rate yields denser data storage and thus greater total data capacity for a magnetic medium.
A key determinant of the current reversal switching time duration is the head swing voltage, i.e. the voltage difference between the head nodes of the magnetizing current control circuit. The larger the voltage drop applied in the opposite direction across the inductive coil after a switching to reverse the current therethrough, the quicker the change in direction of current through the inductive coil. This is because the voltage-current characteristic of an inductive coil is determined by V=Ldi/dt+R.sub.L I, where V is the voltage across the inductive coil, di/dt is the rate of change of current over time through the inductive coil, L is the inductance of the inductive coil, R.sub.L is the resistance of the inductive coil, and I is the current through the inductive coil. Because the inductance of the inductive coil is constant and the resistance of the inductive coil is relatively small, there is a direct relationship between the voltage impressed across the inductive coil after switching and the rate of change of current over time through the inductive coil.
In typical magnetizing current control circuits, the head swing voltage is equal to the voltage difference between the emitters of the first and second switching transistors. In order to create a large voltage difference between the emitters of the first and second switching transistors after a switching to reverse the current through the inductive coil, a similarly large voltage difference is applied to the bases of the first and second switching transistors. After the direction of current changes through the inductive coil, the voltage difference between the emitters of the first and second switching transistors (which is equal to the head swing voltage) decreases to nearly zero while the voltage difference between the bases of the first and second switching transistors remains constant. As a result, the remainder of the large voltage difference between the bases of the first and second switching transistors is compensated by the base-emitter pn junction of either the first or second switching transistor. However, the base-emitter pn junctions of the switching transistors have a low breakdown voltage. Exceeding the breakdown voltage allows a reverse current to flow through these base-emitter pn junctions, and over time sufficiently degrades the current gain of the switching transistors, thereby reducing current to the magnetic recording head and ultimately the magnetic strength of the data it records to a magnetic medium. To avoid exceeding the breakdown voltage of these base-emitter pn junctions it is typically necessary to sacrifice switching rate by limiting head swing voltage (e.g. limiting the voltage difference between the first and second electrical power sources).
Accordingly, there is a need for a magnetizing current control circuit that prevents breakdown of the first and second switching transistors without diminishing the head swing voltage.